1. Field of the Invention
The invention generally relates to a thermophoretic plate having a low emissivity surface and, more particularly, to a thermally insulated thermophoretic plate with a uniformly distributed emissivity surface used in a lithographic tool.
2. Background Description
A lithographic tool uses many components such as, for example, reticles and optical subsystems to ensure precise image transfer onto a wafer to produce a desired microelectronic device. But, the ability to produce high quality microelectronic devices and reduce yield losses is dependent upon maintaining the surfaces of critical components substantially defect-free. This would include, for example, maintaining the surfaces free of particulate matter, e.g., maintaining an ultra clean surface ensuring that particulate matter is not deposited on the surface of the wafer, the reticle or mask or other critical components. This is of particular concern as finer features are required on the microelectronic device.
The concern about defects caused by particle deposition onto surfaces is of particular importance for the next generation of lithography tools, for example, extreme ultraviolet (EUV) lithography. But, contamination from particulate matter is exacerbated since EUV lithography does not use a pellicle which is customarily employed to protect the reticles (masks) from particle deposition. A traditional pellicle cannot be used with EUV lithography for the following reasons, for example.                (i) A traditional pellicle would cause deleterious effects on the beam of high energy incident radiation used in the EUV lithography techniques. That is, for example, the pellicle may absorb some of the radiation, resulting in a loss of throughput.        (ii) A traditional pellicle made of organic material may decompose under the influence of high energy radiation, thus contributing to the deposition of particulate matter on the critical components.        
In order to use EUV lithography effectively in the absence of a pellicle, it is important to devise alternative schemes for protecting the lithographic surfaces, e.g., critical components such as the reticles, from deposition of particulate matter. Some methods that have been devised to address particulate matter control on EUV components are debris shields through which the incoming EUV radiation is passed to catch or filter the particles, electrostatic fields, and thermophoresis.
Debris shields consist of a mesh or grid covered by a very thin film of material which is relatively transparent to EUV radiation, as well as being resistant to damage by the radiation. Zirconium thin films are an example of such material. The grid support allows the film to be very thin, thereby avoiding significant absorption of the EUV radiation. However the EUV radiation is absorbed by the grid, so the shield must be located far enough from the reticle that the shadows created by the grid are defocused in the reticle plane. This enables the illumination at the reticle to be uniform.
Debris shields are effective to a certain extent, but in an effort to maximize photon illumination, the “mesh” size has to be a compromise between particle pass-through rate and reduction in EUV power. The use of electrostatic fields, on the other hand, relies on the electric charge created on the particle by the presence of the EUV radiation. If the electrostatic field has a strong intensity gradient, uncharged particles may be deflected as well, but the force on the uncharged particle is quite weak. Thus, areas not illuminated by EUV radiation are substantially unprotected.
Thermophoresis represents a force on particles which relies on the presence of a thermal gradient in a gas between the reticle and a thermophoretic plate. In this technique, thermophoretic forces are capable of overcoming particle deposition due to, for example, electrostatic forces, inertia, gravity and other forces. The thermophoretic forces cause particles to be driven from regions of higher gas temperature to regions of lower gas temperature.
Thus, using the principles of thermophoresis, particles located between the reticle and thermophoretic plate are subject to the thermophoretic forces, pulling the particle away from the reticle, which is at a higher temperature than the thermophoretic plate. By using such a technique, the particles will not deposit on the reflective surface, e.g., reticle, and will not degrade the device or result in loss of yield. But, in currently explored thermophoretic systems, distortion of the exposure apparatus can occur due to expansion and contraction resulting from heat transfer throughout the system. In one particular instance, the apparatus may distort due to the temperature difference between the thermophoretic plate and the apparatus thus resulting in printing errors and yield loss.